Three-dimensional printing (3DP), described in U.S. Pat. No. 5,204,055 (incorporated herein by reference), has proven to be useful in creating structures for a variety of purposes including medical applications such as bone substitutes and tissue scaffolds.
In the basic three-dimensional printing process, a layer of powder has been deposited such as by roller spreading, and then drops of a binder liquid have been dispensed onto the powder layer by techniques related to ink-jet printing. The dispensers have been moved by motion control apparatus and have included raster printing or vector printing, or both, in various combinations. Powder particles have been joined together by the action of the binder liquid. Subsequent powder layers have been sequentially deposited and drops of binder liquid dispensed until the desired three-dimensional object is created. Unbound powder has supported printed regions until the drying of the article and then unbound powder has been removed to leave a printed article or preform.
Binding of the particles has been achieved through any one or more of several mechanisms. One mechanism has been that the binder liquid has sometimes dissolved some of the powder. Then, as the solvent in the binder liquid has evaporated, the material from partially or fully dissolved particles has resolidified so as to form a joined or solid mass of that material. Another mechanism has been that the binder liquid has contained a dissolved binding substance which has been left behind when the volatile part of the binder liquid evaporates, and upon evaporation of the volatile, the dissolved binder substance has solidified around solid particles or solidified such that it is connected to solid particles, thereby binding solid particles together. It has also been possible for both of these effects to occur simultaneously.
Among the materials of interest to be manufactured into articles by 3DP have been polymers. Polymers, especially polymers of medical interest, have tended to require the dispensing of organic solvents from printheads in the 3DP process. A particularly useful solvent has been chloroform, because of the substances which it can dissolve. Organic solvents have tended to be more difficult to dispense from printheads than aqueous solvents, because of their combination of low viscosity and low surface tension. Chloroform in particular, even when it has been successfully dispensed from a printhead, has exhibited further difficulties which relate to how sharp a feature can be created during three-dimensional printing. First of all, chloroform's unusually small surface tension and viscosity have given it extra tendency to spread by capillary action in a powder bed. Additionally, there has been a difficulty associated with the time scale at which chloroform evaporates.
In three-dimensional printing using dissolution-resolidification, there is a dissolution time scale during which the dissolution of powder particles into the dispensed binder liquid solvent occurs, as governed by the physical properties of the solvent and the solute. (For example, the molecular weight of a polymer can have a strong influence on dissolution time.) There is also an evaporation time scale which describes the evaporation of the dispensed binder liquid, or at least the solvent portion of the dispensed binder liquid, at typical three-dimensional printing conditions such as at room temperature. The evaporation time scale is essentially also the time scale for resolidification to occur. In order for resolidification to be able to occur, there has to be sufficient time for an appropriate amount of dissolution to occur prior to evaporation. If the solvent evaporates before there has been sufficient time for dissolution to occur, little binding can be achieved. With chloroform, the dissolution time scale has been longer than desired, relative to chloroform's evaporation time. Accordingly, in order to achieve sufficient dissolution of powder particles during 3DP, it has been necessary to print chloroform at a relatively high saturation parameter, close to unity. In 3DP, the saturation parameter is a ratio which describes how much of the available inter-particle empty space is actually occupied by binder liquid. A high saturation parameter, especially close to or exceeding unity, has accelerated bleeding (migration) of binder in the powder bed. This in turn has degraded dimensional resolution of printed features and has made it more difficult to remove unbound powder. For example, bleeding has resulted in powder particles being stuck to the printed region which are not really desired to be stuck to the printed region. A comparison of a heavily-bled 3DP structure (left) with a lightly-bled 3DP structure (right) is shown in FIG. 1.
Other difficulties associated with the use of chloroform and similar solvents in 3DP have been the exposure of nearby components of the 3DP machine to the vapor of a solvent which is aggressive against many materials, and the exposure of the entire binder liquid supply system to liquid chloroform, and the handling of chloroform vapor, which is toxic.
Another issue in 3DP has been that 3DP tends to require adjustment of printing parameters to values which are unique to a particular powder and a particular solvent or binder liquid being used. If there are many powders or solvents/binders of interest, then significant effort can be required to determine specific printing parameters, i.e. it can be hard to respond quickly to a change in the formulation.
Porous biostructures made of polymer are disclosed in U.S. Pat. No. 6,454,811, which is incorporated herein by reference. However, those structures were made by dispensing liquid chloroform from a printhead, which resulted in problems of bleeding of dispensed liquid in the powder bed, and so those articles did not have the dimensional resolution of the articles of the current invention. In U.S. Pat. No. 6,454,811, the dispensing of the liquid chloroform included using masks with a continuous stream of liquid chloroform, and the dispensing was performed onto a bed containing particles of PLGA and a leachable porogen. While the printed articles of the '811 patent (after leaching of the porogen) had a high porosity such as 90%, they were still basically rigid and could not undergo any significant deformation without breaking. It is likely that the rigidity was largely due to the material properties of PLGA. Nevertheless, if any such article were able to be made so that it were squeezable, that might open up additional surgical applications.
As far as fields other than three-dimensional printing, and not considered to be prior art to inventions disclosed herein, in printing systems which involve toner powders, such as electrophotographic, electrographic, or magnetographic imaging systems, it is known to use solvent vapor fixing (or solvating) as a way to permanently fix the toner powders to the paper, as an alternative to the commonly used methods which involve heat. U.S. Pat. No. 5,834,150 discloses using environmentally acceptable halogenated hydrocarbons for this purpose. However, the use in that patent was to create two-dimensional images, not three-dimensional structures. Solvent vapor fusing has also been used in other applications such as preparation of dental preforms using the vapor of liquid methyl methacrylate monomer in conjunction with acrylic cements, as described in U.S. Pat. No. 5,336,700. However, this has not extended to three-dimensional printing, nor has it involved leaching of a porogen for creation and control of pores. U.S. Pat. No. 5,171,834 discloses molding a part and then exposing it to solvent vapors.
Accordingly, it would be desirable to be able to achieve the best possible dimensional resolution in polymeric parts which have the geometric complexity that requires the use of 3DP. It would be desirable to minimize bleeding during 3DP such as by printing at a low saturation parameter. It would be desirable to minimize the handling of chloroform and similar aggressive solvents and the exposure of machine parts to such solvents. It would be desirable to provide control of porosity. It would be desirable to incorporate multiple material compositions in articles made of organic-solvent-soluble materials. It would be desirable to make polymeric articles by 3DP without having to spend effort adjusting the printing parameters for changes of polymer or binder formulation.
In general, for porous polymeric articles, it would be desirable to be able to make those articles with good control over the size and shape of the porosity, especially at large porosity fractions.
It would be desirable to make a porous article made at least partly of polymer, which may include internal features, which is capable of undergoing significant elastic deformation without breaking. Such squeezability might make surgical installation easier, reduce the need for on-the-spot shaping during surgery, maintain contact pressure against neighboring tissue to promote tissue integration and ingrowth, etc.